The present invention relates to capacitors and more particularly to a double layer capacitor having a polymer electrolyte layer. Capacitors are electronic devices or components which store electrical charge. Capacitors contain two conductive electrodes or plates, each of which stores an opposite charge, separated by an insulator or dielectric. The charge is stored at the surface of the plates at the boundary with the dielectric.
There are several different types of capacitors. A simple plate capacitor typically contains two conducting plates which sandwich an insulating medium. For example, aluminum electrolytic capacitors contain an aluminum foil cathode, a layer of aluminum oxide (dielectric) and electrolytic capacitor paper. The capacitance C of the capacitor is equal to KeA/d, where A is the effective area which is common to the two conducting plates, e is the permittivity of free space, d is the distance separating the two plates, and K is the dielectric constant of the insulating medium.
A double layer capacitor (DLC) has a more complex “sandwich” structure, as shown in FIGS. 1 and 1A (prior art). This type of capacitor 2 is based on highly porous carbon electrodes 6 and 8 which become electrically charged to positive or negative when connected to a voltage source. The charged carbon then attracts oppositely charged ions from the electrolyte solution 10, which move through a paper or semi-permeable membrane separator 12 as necessary to get to the appropriately charged carbon. The ions insinuate themselves into the available surface area of the porous material. The current collectors 4 are electrically conductive media that facilitate the transfer of electrons in and out of the point contact electrodes to the termination media of the capacitor cell. Such an arrangement provides the two features needed for high capacitance: electrodes with very large surface area and charges which are separated by very small distances, since capacitance depends directly on area and indirectly on distance.
A main difference between aluminum electrolytic capacitors and DLCs is the nature of the dielectric and the resulting method of electromotive force. Rather than using aluminum oxide as the dielectric, DLCs utilize the naturally occurring electrical double layer effect as the dielectric. That is, when an electric potential is applied to the electrode, a potential difference is created at the electrode-electrolyte interface. This electrostatic interface consists of a double layer between ions in the electrolyte and the electronic charges on the electrodes. When positively charged electrolyte ions form a layer on the surface of the negative electrode, electrons beneath the surface pair up with them. Similarly, at the positive electrode, holes pair up with negative ions, forming a second electronic double layer that is itself also a capacitor in series with the first capacitor. In other words, double layer capacitors store their energy in the electric field of the electrochemical double layers of the two electrodes.
As in aluminum electrolytic capacitors, DLCs use organic solvents as the electrolytic fluid. The useful temperature range of conventional double layer capacitors is narrower than that of aluminum electrolytic capacitors, about −25 to 70° C. rather than −55 to 125° C. However, they have a much greater capacity per unit volume: 1 compared with only 0.001.
Double layer capacitors also have higher capacitance than aluminum electrolytic capacitors, typically exhibiting a capacitance of 10−2 to 102 Farads, relative to 10−6 to 10−2 Farads. In comparison, primary and secondary batteries have a typical capacitance of 102 to 106 Farads. However, double layer capacitors have significant advantages over Ni—Cd and lead sealed batteries. For example, double layer capacitors do not pollute the environment and can perform significantly more charge/discharge cycles (more than 100,000 cycles compared with 500 cycles for a typical Ni—Cd battery and 200-1000 cycles for a lead sealed battery).
DLCs are used in consumer products, such as for appliance power backup and volatile memory power, in computer electronics in memory power sources and decoupling energy sources, and in industrial and automotive electronics. The ability to produce improved DLCs which are safer and more efficient is thus very desirable.